Add some comments

Author: tanishiking

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -38,7 +38,7 @@ class Compiler {
     List(new CheckUnused.PostTyper) ::  // Check for unused elements
     List(new YCheckPositions) ::    // YCheck positions
     List(new sbt.ExtractDependencies) :: // Sends information on classes' dependencies to sbt via callbacks
-    List(new semanticdb.ExtractSemanticDB.PostTyper) :: // Extract info into .semanticdb files
+    List(new semanticdb.ExtractSemanticDB.PostTyper) :: // Extract info and attach to the tree of the unit file
     List(new PostTyper) ::          // Additional checks and cleanups after type checking
     List(new sjs.PrepJSInterop) ::  // Additional checks and transformations for Scala.js (Scala.js only)
     List(new sbt.ExtractAPI) ::     // Sends a representation of the API of classes to sbt via callbacks
@@ -51,7 +51,7 @@ class Compiler {
     List(new Inlining) ::           // Inline and execute macros
     List(new PostInlining) ::       // Add mirror support for inlined code
     List(new CheckUnused.PostInlining) ::  // Check for unused elements
-    List(new semanticdb.ExtractSemanticDB.PostInlining) :: // Extract info into .semanticdb files
+    List(new semanticdb.ExtractSemanticDB.PostInlining) :: // Attach warnings to extracted SemanticDB and write to .semanticdb file
     List(new Staging) ::            // Check staging levels and heal staged types
     List(new Splicing) ::           // Replace level 1 splices with holes
     List(new PickleQuotes) ::       // Turn quoted trees into explicit run-time data structures

compiler/src/dotty/tools/dotc/reporting/Reporter.scala

@@ -114,7 +114,7 @@ abstract class Reporter extends interfaces.ReporterResult {
   /** All errors reported by this reporter (ignoring outer reporters) */
   def allErrors: List[Error] = errors
 
-  /** All errors reported by this reporter (ignoring outer reporters) */
+  /** All warnings reported by this reporter (ignoring outer reporters) */
   def allWarnings: List[Warning] = warnings
 
   /** Were sticky errors reported? Overridden in StoreReporter. */

compiler/src/dotty/tools/dotc/semanticdb/ExtractSemanticDB.scala

@@ -32,10 +32,23 @@ import dotty.tools.dotc.semanticdb.DiagnosticOps.*
 /** Extract symbol references and uses to semanticdb files.
  *  See https://scalameta.org/docs/semanticdb/specification.html#symbol-1
  *  for a description of the format.
+ *
+ *  Here, we define two phases for "ExtractSemanticDB", "PostTyper" and "PostInlining".
+ *
+ *  The "PostTyper" phase extracts SemanticDB information such as symbol
+ *  definitions, symbol occurrences, type information, and synthetics.
+ *  This phase does not write the information to a .semanticdb file;
+ *  instead, it attaches the SemanticDB information to the top-level tree.
+ *
+ *  The "PostInlining" phase extracts diagnostics from "ctx.reporter" and
+ *  attaches them to the SemanticDB information extracted in the "PostTyper" phase.
+ *  Afterwards, it writes the SemanticDB to a ".semanticdb" file.
+ *  We need to run this phase after the "CheckUnused.PostInlining" phase
+ *  so that we can extract the warnings generated by "-Wunused".
  */
 class ExtractSemanticDB private (phaseMode: ExtractSemanticDB.PhaseMode, suffix: String, _key: Property.Key[TextDocument]) extends Phase:
 
-  override def phaseName: String = ExtractSemanticDB.phaseNamePrefix + suffix
+  override val phaseName: String = ExtractSemanticDB.phaseNamePrefix + suffix
 
   override val description: String = ExtractSemanticDB.description
 
@@ -55,7 +68,7 @@ class ExtractSemanticDB private (phaseMode: ExtractSemanticDB.PhaseMode, suffix:
       unit.tpdTree.putAttachment(_key, extractor.toTextDocument(unit.source))
     else
       unit.tpdTree.getAttachment(_key) match
-        case None => ???
+        case None =>
         case Some(doc) =>
           val warnings = ctx.reporter.allWarnings.collect {
             case w if w.pos.source == ctx.source => w.toSemanticDiagnostic